robotics//drone

A drone, in the sense used across this wiki, is an unmanned rotorcraft that cannot stay in the air without active control, and it is the running example of the closed loop because it has every stage of one and forgives nothing: it is unstable without feedback, its sensors are cheap and noisy, it has little weight to spare for computers, and it must decide in milliseconds. Fixed-wing UAVs share much of the machinery; the notes here mostly mean multirotors.

A drone, in the sense used across this wiki, is an unmanned rotorcraft that cannot stay in the air without active control, and it is the running example of the closed loop because it has every stage of one and forgives nothing: it is unstable without feedback, its sensors are cheap and noisy, it has little weight to spare for computers, and it must decide in milliseconds. Fixed-wing UAVs share much of the machinery; the notes here mostly mean multirotors.

Follow one gust in slow motion. The gust tilts the drone half a degree; its state (position, velocity, orientation and rotation rate on three axes, twelve numbers) has changed. The IMU reports at 1 to 8 kHz, the barometer at tens of Hz, the GNSS receiver at 5 to 10 Hz with tens to hundreds of milliseconds of delay. An extended Kalman filter combines them with a motion model into the best guess of the state and its uncertainty. A cascade of controllers turns I want to be here into I want this tilt, then into I want to rotate this fast, and finally into four motor commands. The propellers push, the drone moves, and the loop starts again.

The airframe as a plant is the multirotor: thrust from propellers, a mixer that shares the work between motors, ESCs that drive them, and the battery whose voltage sets how much thrust a command buys.

The software that closes the loops is the autopilot, with each loop at the rate its dynamics demands, on a Cortex-M flight controller.

To reason about the control with pencil and paper, two worked models strip it down: the vertical drone (height only) and the planar drone, which moves in a vertical plane and already shows why moving sideways requires tilting and why the control is a cascade.

The power budget on a drone is unforgiving. A 1.5 kg quadcopter hovers on about 200 W, so a 15 W companion computer costs some 7 % of the flight time before its weight is counted, and the weight costs more thrust again. Onboard computing, sensors and payload all buy capability with endurance (embedded system).

It is a cyber-physical system in the plainest sense: a computation error becomes a wrong movement in the air, which is why its software is held to deadlines, stability margins and failsafes that office software never meets.